The invention relates to expandable or expanded polymer beads based on biodegradable saturated polyesters.
It is known that some naturally occurring materials, e.g. cellulose and starch, are biodegradable, i.e. are decomposed by microorganisms, in particular during composting, to give low-molecular-weight substances. Some synthetic polymers, for example polyesters, are also biodegradable. Although purely aliphatic polyesters have good biodegradability they have low suitability for practical applications, since they have a low level of mechanical and thermal properties. In contrast, aromatic polyesters have good mechanical properties but are not biodegradable.
WO 96/07687 has disclosed that copolyesters in which the monomer components are firstly aliphatic diols and secondly aliphatic and aromatic dicarboxylic acids have good mechanical and thermal properties and are at the same time biodegradable. This publication also mentions foamed moldings, but says nothing about their production.
WO 96/15173 also describes biodegradable copolyesters made from aliphatic diols and from a mixture of adipic acid and terephthalic acid, which have been modified by incorporating other transesterification components. These polyesters can be used to produce foams, by mixing a polymer melt with blowing agents and extruding the mixture as a foam.
WO 97/43329 relates to biodegradable branched copolyesters with increased melt viscosity containing polyfunctional polyols and, respectively, polycarboxylic acids incorporated by condensation. Again, the copolyesters can be processed to give foams by extruding the melt.
Finally, WO 97/44388 also describes foams made from moisture-resistant biodegradable mixtures of naturally occurring polymers, e.g. starch, with polyesters having hydroxyl functions. These biodegradable materials, too, were foamed by extruding a foam.
A disadvantage of extruding foams is that they can produce only simply shaped foam moldings, e.g. sheets or profiles. It is known that foam moldings of any desired shape can be produced by expanding expandable or expanded polymer beads, e.g. those based on polystyrene and/or on polyolefins, to give a foam and sintering these.
It is an object of the present invention to provide expanded or expandable particles based on biodegradable polyesters. We have found that this object is achieved by means of the processes as claimed in claims 1 and, respectively, 2.
EP-A 601390 describes degradable moldings made from foamed polylactides produced by foaming a mixture of amorphous polylactide pellets comprising blowing agent and finely divided polylactide in which no blowing agent is present. The polylactide pellets comprising blowing agents may be attained by suspending polylactide pellets in water and injecting blowing agents, e.g. pentane, at pressures of up to 100 bar over a period of up to 10 h, at temperatures below 50xc2x0 C. This is a very complicated and time-consuming operation. Moldings made from polylactides are not directly biodegradable. They first have to be hydrolyzed at elevated temperatures to give lactic acid, and this is then further biodegraded.
Polyesters suitable for the novel process are described in claims 3 and 4, and also in the publications which have been cited: WO 96/07687, WO 96/15173, WO 97/43329 and WO 97/44388.
Particularly preferred partly aromatic polyesters include polyesters which contain, as significant components:
a) an acid component made from
a1) from 30 to 95 mol % of at least one aliphatic or cycloaliphatic dicarboxylic acid or ester-forming derivatives of these or mixtures of these,
a2) from 5 to 70 mol % of at least one aromatic dicarboxylic acid or ester-forming derivatives of these, and
a3) from 0 to 5 mol % of a compound containing sulfonate groups, and
b) a diol component made from at least one C2-C12 alkanediold or from a C5-C10 cycloalkanediol or mixtures of these,
and also, if desired, one or more components selected from the group consisting of
c) a component selected from the group consisting of
c1) at least one dihydroxy compound of the formula I containing ether functions
HOxe2x80x94[(CH2)nxe2x80x94O]mxe2x80x94Hxe2x80x83xe2x80x83(I)
xe2x80x83where n is 2, 3 or 4 and m is an integer from 2 to 250,
c2) at least one hydroxycarboxylic acid of formula IIa or IIb
HOxe2x80x94[xe2x80x94C(O)xe2x80x94Txe2x80x94Txe2x80x94]pHxe2x80x83xe2x80x83(IIa)

xe2x80x83where p is an integer from 1 to 1500 and r is an integer from 1 to 4, and G is a radical selected from the group consisting of phenylene, xe2x80x94(CH2)qxe2x80x94, where q is an integer from 1 to 5, xe2x80x94C(R)Hxe2x80x94 and xe2x80x94C(R)HCH2, where R is methyl or ethyl,
c3) at least one amino-C2-C12 alkanol or at least one amino-C5-C10 cycloalkanol or mixtures of these,
c4) at least one diamino-C1-C8-alkane,
c5) at least one 2,2xe2x80x2bisoxazoline of formula III 
xe2x80x83where R1 is a single bond, (CH2)2-alkylene, where z=2, 3 or 4, or phenylene, and
c6) at least one aminocarboxylic acid selected from the group consisting of the naturally occurring amino acids, polyamides with a molar mass of not more than 18000 g/mol obtainable by polycondensing a dicarboxylic acid having from 4 to 6 carbon atoms and a diamine having from 4 to 10 carbon atoms, compounds of the formulae IVa and IVb
xe2x80x83HOxe2x80x94[xe2x80x94C(O)xe2x80x94Txe2x80x94N(H)xe2x80x94]sHxe2x80x83xe2x80x83(IVa)

xe2x80x83where s is an integer from 1 to 1500 and t is an integer from 1 to 4, and T is a radical selected from the group consisting of phenylene, xe2x80x94(CH2)nxe2x80x94, where n is an integer from 1 to 12, xe2x80x94CR2Hxe2x80x94 and xe2x80x94CR2HCH2xe2x80x94, where R2 is methyl or ethyl,
xe2x80x83and polyoxazolines with the repeat unit V 
xe2x80x83where R3 is hydrogen, C1-C6-alkyl, C5-C8-cycloalkyl, phenyl, unsubstituted or substituted with up to three C1-C4-alkyl groups, or tetrahydrofuryl,
xe2x80x83or mixtures of c1 to c6,
and
d) a component selected from the group consisting of
d1) at least one compound having at least three groups capable of ester formation,
d2) at least one isocyanate,
d3) at least one divinyl ether,
or mixtures made from d1) to d3).
The acid component a in the partly aromatic polyesters comprises from 30 to 70 mol %, in particular from 40 to 60 mol %, of al and from 30 to 70 mol %, in particular from 40 to 60 mol %, of a2.
Possible aliphatic and, respectively, cycloaliphatic acids and the corresponding derivatives al are those mentioned above. Particular preference is given to adipic acid and sebacic acid, the ester-forming derivatives of each of these, or mixtures of these. Particular preference is given to adipic acid and its ester-forming derivatives, such as its alkyl esters or mixtures of these.
Aromatic dicarboxylic acids a2 which should be mentioned are generally those having from 8 to 12 carbon atoms, preferably 8 carbon atoms. Examples which may be mentioned are terephthalic acid, isophthalic acid, 2,6-naphthoic acid and 1,5-naphthoic acid, and also ester-forming derivatives of these, particularly the di-C1-C6-alkyl esters, e.g. dimethyl, diethyl, di-n-propyl, diisopropyl, di-n-butyl, diisobutyl, di-tert-butyl, di-n-pentyl, diisopentyl and di-n-hexyl esters. Other suitable ester-forming derivatives of the dicarboxylic acids of a2 are the anhydrides.
However, it is in principle also possible to use aromatic dicarboxylic acids a2 having a large number of carbon atoms, for example up to 20 carbon atoms.
The aromatic dicarboxylic acids and their ester-forming derivatives a2 may be used individually or as a mixture of two or more of these. Particular preference is given to terephthalic acid and its ester-forming derivatives, such as dimethyl terephthalate.
The polyesters preferably melt at from 70xc2x0 to 150xc2x0 C., in particular from 80xc2x0 to 130xc2x0 C. The saturated, partly aromatic polyesters under consideration here do not have precisely defined melting points, but rather a melting range. The impregnation according to the invention with the blowing agent should preferably be undertaken within this melting range.
The partly aromatic polyesters are characterized by a molar mass (Mn) of from 5000 to 100000 g/mol, in particular from 10000 to 40000 g/mol, with a viscosity number of from 50 to 400 ml/g, in particular from 100 to 300 ml/g (measured in o-dichlorobenzene/phenol, weight ratio of 50/50, at a concentration of 0.5% by weight of polymer at 25xc2x0 C.).
The biodegradable polyesters may be blended with up to 200%, based on their weight, of other biodegradable polymers. Examples which should be mentioned are: cellulose and cellulose derivatives, starch and starch derivatives, polyhydroxyalkanoates, such as polyhydroxybutyrate and polylactides.
The biodegradable polymers may comprise additives, which may be incorporated during the polymerization procedure or subsequently, for example into a melt with the biodegradable polymers. Examples of additives are stabilizers, neutralizing agents, lubricants, mold-release agents, antiblocking agents, nucleating agents, non-fluorescing dyes and fillers.
From 0 to 80% by weight of additives may be added, based on the biodegradable polymers. Examples of suitable additives are carbon black, powdered lignin, cellulose fibers, natural fibers, such as sisal or hemp, iron oxides, clay minerals, ores, calcium carbonate, calcium sulfate, barium sulfate and titanium dioxide, stabilizers, such as tocopherol (Vitamin E), organic phosphorus compounds, mono-, di- or polyphenols, hydroquinones, diarylamines and thioethers. Examples of nucleating agents are talc, carbon black and graphite. Other additives which may be present are lubricants and mold-release agents based on hydrocarbons, on fatty alcohols, on higher carboxylic acids, on metal salts of higher carboxylic acids, such as calcium stearate or zinc stearate, or on montan waxes. These stabilizers, etc. are described in detail in Kunststoff-Handbuch, Vol. 3/1, Carl-Hanser Verlag, Munich, 1992, pp. 24-28.
The preparation of the expanded or expandable polymer beads starts from polyester pellets whose average diameter is preferably from 0.2 to 10 mm, in particular from 0.5 to 5 mm. 100 parts by weight of these pellets are dispersed in from 100 to 500 parts by weight of suspension medium in a stirred reactor.
The densities of the polymer pellets and of the suspension medium should be approximately matched to one another to ensure good distribution of the pellets in the stirred liquid. Since the density of the preferred polyesters is generally about 1.25 g/cm3, the density of the liquid should be above 1.0 g/cm3, preferably from 1.05 to 1.3 g/cm3 and in particular from 1.1 to 1.25 g/cm3. Examples of liquids are ethylene glycol and glycerol, and also mixtures of these with water. Another way of matching the densities is to use water as the liquid and use polymer pellets which have been foamed to a slight extent. The latter can be produced by mixing a melt of the polymer with a chemical blowing agent, e.g. azodicarbonamide or bicarbonate/citric acid, extruding the melt and pelletizing.
The dispersion may comprise from 0.01 to 5% by weight, based on the polymer pellets, of conventional suspension stabilizers, such as water-insoluble inorganic compounds, for example metal carbonates, metal phosphates or metal oxides, which are used together with surface-active substances, e.g. sulfonates or ethoxylates. If the density of the polymer pellets are higher than that of the suspension medium, suspension stabilizers can be dispensed with.
The blowing agents used may comprise either organic liquids or inorganic gases or mixtures of these. Possible liquids are halogenated hydrocarbons, but preference is given to saturated aliphatic hydrocarbons, in particular those having from 3 to 8 carbon atoms. n-Butane is particularly preferred. Suitable inorganic gases are nitrogen, air, ammonia and carbon dioxide. Ethanol is also suitable.
From 1 to 50 parts by weight, in particular from 3 to 30 parts by weight, of the blowing agent are used, based on 100 parts by weight of polymer. The blowing agent here may be added prior to or during the heating of the reactor contents.
During impregnation, the temperature should be above the softening point of the polymer. It may be from 40xc2x0 C. below to 40xc2x0 C. above the melting point (crystalline melting point from DSC), but should preferably be below the melting point.
Depending on the nature and amount of the blowing agent, and also on the temperature, the pressure arising in the reactor is generally higher than 2 bar and not more than 50 bar.
The impregnation has generally been completed after from 30 to 300 min.
When producing expanded polymer particles, the pressure on the dispersion is released at an elevated temperature, usefully at the impregnating temperature or somewhat below this.
In this case, the pellets impregnated with blowing agent expand to give foam beads with a diameter of from 1 to 20 mm, which are isolated from the suspension medium, washed and dried. Depending on the impregnation conditions, their bulk density may be from 10 to 300 g/l.
Expandable polymer beads are obtained if, after impregnation but before reducing the pressure, the dispersion is cooled to a large extent, usefully at room temperature. This gives polymer beads which comprise blowing agent but have not been expanded to give foam. These are stored and transported more readily than foam beads. They may be expanded by conventional methods, e.g. using steam or hot air, to give foam beads.
The novel foam beads may be further foamed and sintered as usual in closed molds, using steam or hot air, to give foam moldings with any desired shape.
Foam moldings made from the biodegradable foam beads may be used as disposable kitchenware or as packaging blocks, dishes or cups. These can be composted and thus degraded once they have been comminuted.
In the examples, all of the parts and percentages given are by weight.